Is it safe to erase the system data of my android tablet (not rooted)?
Yes, it's safe to Factory Reset your tablet.
If I restore the factory setting of my Android, will the uninstallable
apps be also removed or only those that can be uninstalled?
Only those that can be uninstalled will be removed. Doing a Factory Reset is like making your phone brand new. You will lose all your installed apps and all your data, but you will still have the stock / default apps that your phone came with.
Once the phone reboots after a Factory Reset, you start from scratch. You will log-in to your Google account, and have the option of restoring the apps you installed and bought before from Google Play.